Results 1 - 10 of 4756
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[en] Predictions of the recently developed paleoclassical transport model are compared with data from many toroidal plasma experiments: electron heat diffusivity in DIII-D, C-Mod and NSTX ohmic and near-ohmic plasmas; transport modeling of DIII-D ohmic-level discharges and of the RTP ECH 'stair-step' experiments with eITBs at low order rational surfaces; investigation of a strong eITB in JT-60U; H-mode Te edge pedestal properties in DIII-D; and electron heat diffusivities in non-tokamak experiments (NSTX/ST, MST/RFP, SSPX/spheromak). The radial electron heat transport predicted by the paleoclassical model is found to agree with a wide variety of ohmic-level experimental results and to set the lower limit (within a factor ∼ 2) for the radial electron heat transport in most resistive, current-carrying toroidal plasmas -- unless it is exceeded by fluctuation-induced transport, which often occurs in the edge of L-mode plasmas and when the electron temperature is high (∼>Tecrit ∼B2/3(bar α)1/2 keV) because then paleoclassical transport becomes less than gyro-Bohm-level anomalous transport
[en] Externally imposed non-axisymmetric magnetic perturbations are observed to alter divertor heat and particle flux profiles in the National Spherical Torus Experiment (NSTX). The divertor profiles are found to have a modest level of multiple local peaks, characteristic of strike point splitting or the 'magnetic lobe' structure, even before the application of the 3D fields in some (but not all) NSTX discharges. This is thought to be due to the intrinsic error fields. The applied 3D fields augmented the intrinsic strike point splitting, making the amplitude of local peaks and valleys larger in the divertor profile and striations at the divertor surface brighter. The measured heat flux profile shows that the radial location and spacing of the striations are qualitatively consistent with a vacuum field tracing calculation. 3D field application did not change the peak divertor heat and particle fluxes at the toroidal location of measurement. Spatial characteristics of the observed patterns are also reported in the paper.
[en] This overview is an assessment of the gyrokinetic framework and simulations to compute turbulent transport in fusion plasmas. It covers an introduction to the gyrokinetic theory, the principal numerical techniques which are being used to solve the gyrokinetic equations, fundamentals in gyrokinetic turbulence and the main results which have been brought by simulations with regard to transport in fusion devices and fluctuation measurements. (topical review)
[en] A new mechanism for the fast excitation of the energetic geodesic acoustic mode (EGAM) is proposed to explain the recent experiment in DIII-D (Nazikian et al 2008 Phys. Rev. Lett. 101 185001), where the mode turns on in less than a millisecond after the turn-on of the neutral beam injection. The existence of loss boundary in pitch angle when beam particles are injected counter to the plasma current is crucial to the formation of negative energy EGAM mode. The resonance of this negative energy wave with energetic particles, whose distribution decreases with energy, destabilizes the mode. We find that when there is a loss region, the onset time of instability can be significantly shorter than it would be if the injected particles had no loss region.
[en] A parametric model for fusion neutron emissivity is presented and applied to the KN3 neutron camera data collected during the trace tritium experiment at the Joint European Torus. This work is aimed at achieving a good compromise between accuracy of tomographic reconstruction and low model complexity. This means low numerical degeneracy and good time consistency of the results. The model is compared both with plasma simulation codes and other tomographic techniques, which use KN3 line integrated emissivity data, showing good agreement over the entire data set analysed (∼500 plasma discharges).
[en] Collective Thomson scattering (CTS) experiments were carried out at ASDEX Upgrade to measure the one-dimensional velocity distribution functions of fast ion populations. These measurements are compared with simulations using the codes TRANSP/NUBEAM and ASCOT for two different neutral beam injection (NBI) configurations: two NBI sources and only one NBI source. The measured CTS spectra as well as the inferred one-dimensional fast ion velocity distribution functions are clearly asymmetric as a consequence of the anisotropy of the beam ion populations and the selected geometry of the experiment. As expected, the one-beam configuration can clearly be distinguished from the two-beam configuration. The fast ion population is smaller and the asymmetry is less pronounced for the one-beam configuration. Salient features of the numerical simulation results agree with the CTS measurements while quantitative discrepancies in absolute values and gradients are found.
[en] The nonlinear dynamics of a two-dimensional (2D) model for collisionless magnetic reconnection is investigated both numerically and analytically. For very low values of the plasma β, parallel magnetic perturbations tend to be proportional to the vorticity perturbations, but as β increases, detachment of these quantities takes place. The subsequent difference between the structure of the vorticity and the parallel magnetic perturbations can be explained naturally in terms of the 'normal' field variables that emerge from the noncanonical Hamiltonian theory of the model. A three-dimensional extension of the reconnection model is also presented, its Hamiltonian structure is derived, and the corresponding conservation properties are compared with those of the 2D model. A general method for extending a large class of 2D fluid plasma models to three dimensions, while preserving the Hamiltonian structure, is then presented. Finally, it is shown how such models can also be extended, while preserving the Hamiltonian structure, to include externally applied fields, that can be used, for instance, for modelling resonant magnetic perturbations.
[en] The influences of resonant magnetic perturbations (RMPs) on the poloidal rotation at the edge of a tokamak are investigated. Specific results are displayed for the tokamak TEXTOR with the dynamic ergodic divertor (DED). The latter can be operated in three different base mode configurations, namely 12/4, 6/2 and 3/1. The base mode configurations distinguish themselves by resonating with different island chains and having distinctly different penetration depths. Calculations predict a strong influence of the DED base mode configurations on the strength of the poloidal plasma rotation. The interpretation of the results emanates from the electron and ion drift motions in partially stochastic magnetic fields. Generally, RMPs cause incomplete magnetic chaos; the latter influences the drift motion of electrons and ions differently. By virtue of the formed ambipolar electric field, the poloidal plasma rotation is directly connected via the radial force balance. With increasing current in the DED perturbation coils the electron and ion last closed drift surfaces as well as internal drift surfaces break up differently for each species. These break-ups, as well as the changes in the poloidal rotation in dependence on the electron and ion temperatures, are investigated in detail.
[en] Electron Bernstein wave (EBW) assisted plasma current start-up has been demonstrated for the first time in a tokamak. It was shown that plasma currents up to 17 kA can be generated non-inductively by 100 kW of RF power injected. With optimized vertical field ramps, plasma currents up to 33 kA have been achieved without the use of solenoid flux. It is shown that the plasma formation and current generation are governed predominantly by EBW current drive. Experimental results are consistent with ray-tracing and quasilinear Fokker-Planck modelling. (letter)